COMP: Fix NIFTI CMake variable separation warning.

For:

Syntax Warning in cmake code at

    /home/kevin/kitware/ITK/Modules/ThirdParty/NIFTI/src/nifti/CMakeLists.txt:103:38

Argument not separated from preceding token by whitespace.

Change-Id: If87b17603a808caaff4fb307bf797df007bb0d50